1. Field of the Invention
This invention relates to motors having an auxiliary start/capacitor run winding and, more specifically to a system for removal of the PTC motor starter from the circuit during motor operation.
2. Brief Description of the Prior Art
Permanent split capacitor (PSC) electric motors are generally provided with a stator winding circuit which includes a main or run winding and an auxiliary or start winding connected in series with a run capacitor which are both connected in parallel with the main winding. During motor starting and running, both the main winding and the start winding are connected across the motor power source to magnetically excite the rotor and cause rotation thereof. Sometimes a low resistance positive temperature coefficient (PTC) resistor starter is placed in parallel with the run capacitor to increase the starting torque of the motor. When the motor has reached a predetermined speed or after a predetermined time, it is advantageous to disconnect the PTC starter from the circuit to save the energy wasted keeping the PTC hot in its high resistance off state.
In some of the older prior art, the start winding was removed from the circuit by some mechanical speed related device. More recent art, as exemplified, for example by Rathje U.S. Pat. No. 4,161,681 and DeFilippis et al. U.S. Pat. No. 4,706,152 minimized the effect of the start winding by placing a resistor in series with the start winding having a positive temperature coefficient of resistance. The PTC resistor self heats due to current therethrough, this heat causing an increase in the resistance thereof, thereby causing a gradual decrease in the current through the start winding to that current primarily through the run capacitor with continued motor operation. The resistor is designed to reach a predetermined maximum resistance in a predetermined time period which is related to the motor design.
A problem with such PTC resistor controlled start windings is that the current through the PTC resistor is never completely terminated and wastes energy. This is readily apparent from the fact that, if current were completely cut off, the PTC resistor would start to cool off, thereby causing a decrease in its resistance and an increase in the current therethrough. It follows that an equilibrium low current through the PTC resistor is established during standard motor operation and after the initial start. This continual current through the PTC resistor causes unnecessary heating in the PTC resistor as well as undesirable power dissipation with concomitant loss of motor system efficiency.